Electric resistance and method of making



Patented Oct. 3, 1950 ELECTRIC RESISTANCE AND METHOD or MAKING EvertJohannes Willem Verwey and Marinus Gerard van Eruggen, Eindhoven,Netherlands, assignors, by mesne assignments, to Hartford National Bankand Trust Company, Hartford,

Conn., as trustee No Drawing. Application January 14, 1946, Serial No.641,181. In the Netherlands August 22, 1941 Section 1, Public Law 690,August 8, 1946 Patent expires August 22, 1961 7 Claims.

For manufacturing electric resistances having a negative temperaturecoefiicient various combinations of F6304 with double oxides having aspinel structure similarly to F6304 have been suggested.

It has now been found that the combination FeOFe2O3-MgO-Al2O3 is ofparticular advantage provided the initial substances are converted intoa sintered material consisting of a uniform spinel phase and this isdone in such manner that at a temperature of about 500 C. the materialis not supersaturated with a second phase. For this, it is generallynecessary that the molecular ratio between the total quantities ofbivalent and trivalent oxides is not different from 1 or only to aslight extent.

If this condition is satisfied a second phase which would causeundesired variations of the resistance cannot separate out at operatingtemperatures in excess of 500 C. As far as the region below 500 C. isconcerned it should be mentioned that below temperatures of this orderof magnitude the velocity of separation of a second phase is very lowwith a resistance material according to the invention. i

In accordance with the foregoing it is necessary according to theinvention so to choose the conditions of sintering, to wit thetemperature and the partial pressure of the oxygen in the surroundinggas, that the molecular ratio FeO:Fe2O3 required for the saiddesideratum is adjusted in the material. In order that during thesubsequent cooling this ratio may be preserved to the best possibleextent cooling may be effected rapidly with the simultaneous replacementof the oxygenous atmosphere by an inert gas or both the heating and therapid cooling may be effected in an atmosphere in which the partialpressure of the oxygen decreases when the temperature falls, for examplein a mixture of nitrogen and water vapor.

The specific resistance of the resistors according to the invention issolely governed by the mixing ratio of the constituents and primarily bythe content of iron oxide in the spinel form. Generally, this contentwill be less than 50 mol. per cent for obtaining values of practicalusefor the specific resistance.

The resistance material according to the invention offers the advantagethat at comparative- 1y high load chemical conversions (for exampleseparation of oxygen when on load in a vacuum orin an inert gas,oxidation under the influence of the air oxygen) that would result invariation of the resistance value do not occur. This favourable propertyis apparently governed by the presence of magnesia spinel which byitself is not oxidisable or reducible and has such a high melting pointthat at high temperatures the velocity of the diffusion and of thereaction of the atoms of the mixed crystal containing FeO+Fe2Os is ofslight magnitude.

By reason of the particular, limited field of compositions of thequaternary system according to the invention the additional advantage isachieved, as indicated before, that at high load temperatures avariation in the resistancev With the same resistance value the meltingI point of the material may slightly vary in accordance with thecomposition. The materials having the highest melting point and beingchemically most resistant; are obtained when the molecular ratioMgO-:A12O3 is in the neighbourhood of '1. In that case also a molecularratio FeO:Fe2O3 in the neighbourhood of 1 is necessarily desired.Variations in the partial pressure of the oxygen in the sinteringatmosphere, as may occur dur ing manufacture, produce but slightdifferences,

in the value of the specific resistance with materials thus composed andthis results technically in the advantage of high reproducibility insintering.

Since the ratio FeOzFezOa is controlled by the choice of the gasatmosphere and the sintering temperature any form of iron oxide and, ifdesired iron powder may be used as the initial material for theresistances according to the invention; I

the MgO-I-AlzOs may be used wholly or in part as magnesia spinel. Inaddition, the oxides concerned may be isomorphously replaced for a smallpart by other oxides. at very high temperature it may be desirable forIf an initial mixturesinters 3 example for reasons of ease ofmanufacture that a small quantity of a sintering agent should be used.

The resistances according to the invention may be used for example forthe elimination of voltage pulses and as compensation resistances.

In order that the invention may be clearly understood and readilycarried into effect it will now be described more fully with referenceto two practical examples.

Example I MgO and A1203 in a molecular ratio 1:1 fired at about 1000 C.and Fe203 is ground with water in an iron ball mill. The composition ofthe initial mixture is so chosen that a material is obtained in whichthe ratio of the number of gramme-molecules Mg+Al203 (calculated asMgAl204) to the number of molecules of iron oxide (calculated as Fe304)is 2.5. The material is then moulded with the addition of an organicbinder to rods of 7 mms. in diameter and 8 mms. in length and sinteredfor minutes at 1570 .C. (black-body temperature, i. e. the equivalenttemperature of an ideal heat-radiator, a black body) in an atmosphere ofnitrogen passed through waterof C.. The sintering operation is carriedout in an electric furnace in avessel of thoria, which material does notreact to any appreciable extent with the resistance material. Subsequentto sintering cooling is efiected rapidly by pushing the vessel into acold part of the furnace.

The resistance value at ordinary temperatures of a resultant rod is25000. At a load with 100-ma. of such a resistance rod which'is providedwith nickel leading-in wires of 0.5 mm. in thickness the voltage acrossthe resistance is 13 volts (resistance 1300). At a load with 6 watts thetemperature at the centre of the rod mounts to about 500 C., whilst atthe same time the resistance value decreases to 130. At this very highload in air for 24 hours the resistance value remains unvaried with ameasuring accuracy of 1%.

Erample II Except for the ratio MgA1204:Fe304 which in the present caseis 2:1 the composition of the initial mixture is chosen to be quiteidentical with that of Example I. This material is then moulded with theuse of an organic binder to tubes of 12 mms. ininternal diameter and 16mms. in external diameter. These tubes are suspended in a verticallyarranged furnace and sintered for 30 minutes at 1540 C. in an atmosphereof nitrogen containing 1% of oxygen. The oxygenous atmosphere is nextreplaced by pure dry nitrogen and the tubes are rapidly cooled bydropping them after about 30 seconds into a cold part of the furnace.After internally and externally silverplating and dividing of the tubesinto lengths of 40 mms. resistances of a value of 100 at 30 C. and of1.80 at 100 C. are obtained.

What we claim is:

1. A negative temperature coefficient electrical resistance elementconsisting of a sintered mass of mixed crystals of FeO, Fe203, MgO, andA1203, in which the molecular ratio of is approximately unity and whichcontains less than mol. per cent of iron oxide in spinel form (Fe304),said sintered mass having a uniform spinel phase which is less thansupersaturated wi(t)h a second phase at a temperature of about 50 C.

2. A negative temperature coeificient electrical resistance elementconsisting of a sintered mass of mixed crystals of FeO, Fe203, MgO, andA1203, in which the molecular ratio of MgOzAlzOz is approximately unityand which contains less than approximately 50 mol. percent of iron oxidein spinel form (F6304), said sintered mass having a uniform spinel phasewhich is less than supersaturated with a second phase at a temperatureof about 500 C.

3. A negative temperature coefficient electrical resistance elementconsisting of a, sintered mass of mixed crystals of Fe0, Fe20s, MgO, andA1203, in which the molecular ratios of MgOzAlzOa and of Fe0:Fe203 arerespectively approximately unity and which contains less thanapproximately 50 mol. per cent of iron oxide in spinel form (Fe304),said sintered mass having a uniform spinel phase which is less thansupersaturated with a second phase at a temperature of about 500 C.

4. A negative temperature coefficient electrical resistance elementconsisting of a sintered mass of mixed crystals of FeO, Fe2O3, MgO, andA1203, in which the molecular ratios of MgOzAlzOa and FeOzFezOs,respectively, are approximately unity and the gram-molecular ratioMgO+A1203 2F60+F8203 is approximately 2.5:1, said sintered mass having auniform spinel phase which is less than supersaturated with a secondphase at temperatures of about 500 C.

5. A negative temperature coefiicient electrical resistance elementconsisting of a sintered mass of mixed crystals of FeO, Fe203, .MgO, andA1203, in .which the molecular ratios of Mg01A1203 and FeOzFezOz,respectively, are approximately unity and the gram-molecular ratio of(MgO-i-AlzOs) :(FeO +Fe2O3) is approximately 2:1, said sintered masshaving a uniform spinel phase which is less than supersaturated with -asecond phase at a temperature of about 500 C.

6. A method of manufacturing an electrical resistance element having anegative temperature coeflicient of resistance comprising the steps ofmixing powdered MgO and A1203 in the molecular ratio of approximately1:1 and Fe203 computed on the basis of Fe304 in the gram-molecular ratioof 2.5:1 between the MgO and A1203 and the Fe203, and sintering themixture at a temperature of approximately 1570 C. in a nitrogenatmosphere substantially free of oxygen to form a sintered mass of mixedcrystals of MgO, A1203, FeO, and Fe203 in which the molecular ratios ofMgOZAlzOs and Fe0:Fe203 are approximately unity and which contains lessthan 50 mol. percent of iron oxide in spinel form.

7. A method of manufacturing an electrical resistance element having anegative temperature coefiicient of resistance comprising the steps ofmixing powdered MgO and A1203 in the molecular ratio of approximately1:1 and Fe203 computed on the basis of Fe3O4 in the grammolecular ratioof 2:1 between the MgO and A1203 and the Fe2O3, sintering the mixture ata temperature of approximately 1540" C. in an atmosphere of nitrogenwhich is substantially free of oxygen, and cooling the sintered massthus formed in an atmosphere 0f nitrogen to form a sintered mass ofmixed crystals of FeO, Fe203, MgO and A1203, in which the molecular 5ratios of MgOZAlzOa and FeO:Fe2O3 are approximately unity and the ironoxide in spinel file of this patent: 10 523,051

6 UNITED STATES PATENT Name Date Steinmetz July 23, 1907 Andres Jan. 14,1936 Haglund July 21, 1936 Biefeld. Feb. 27, 1945 FOREIGN PATENTSCountry Date Great Britain July 4, 1940

1. A NEGATIVE TEMPERATURE COEFFICIENT ELECTRICAL RESISTANCE ELEMENTCONSISTING OF A SINTERED MASS OF MIXED CRYSTALS OF FEO, FE3O3, MGO, ANDAL2O3, IN WHICH THE MOLECULAR RATIO OF @SP (MGO+FEO):(AL2O2+FE2O3) @SPIS APPROXIMATELY UNITY AND WHICH CONTAINS LESS THAN 50 MOL. PER CENT OFIRON OXIDE IN SPINEL FORM (FE3O4), AND SINTERED MASS HAVING A UNIFORMSPINEL PHASE WHICH IS LESS THAN SUPERSATURATED WITH A SECOND PHASE AT ATEMPERATURE OF ABOUT 500*C.